The present invention relates generally to current sources. More particularly, the present invention relates to current references that provide a substantially constant source of current.
Current references are circuits designed to provide a source of substantially constant current, typically used in turn by other circuits which depend upon a minimal variance in the supply of current. In fact, the ultimate performance of a circuit which makes use of a current reference is often dependent on the stability of the reference.
One problem with current reference circuits is that the current provided may be sensitive to voltage, temperature, and process variations. Thus, as supply or bias voltage, temperature, or process parameters (such as transistor threshold voltages) vary, the current generated by the reference may also vary. Thus, sensitivity to temperature and power supply voltage variations in current references, and the reduction thereof, has been the subject of much study. See, for example, Sueng-Hoon Lee and Yong Jee, xe2x80x9cA Temperature and Supply Voltage Insensitive CMOS Current Reference,xe2x80x9d IEICE Trans. Electron., Vol. E82-C, No.8, August 1999; and Cheol-Hee et al., xe2x80x9cA Temperature and Supply Insensitive CMOS Current Reference Using a Square Root Circuit,xe2x80x9d IEEE ICVC, October 1997, pp 498-500.
Sensitivity to process variations has been handled historically by using appropriate design margins. For example, if the current generated by the reference changes by a factor of two over the range of expected variations in a process, the current reference manufactured using that process is typically designed to provide a nominal current equal to twice the minimum specified value, so that under worst case conditions the minimum current value is guaranteed to exist. However, power provided to the reference is usually wasted as a result, in part because the nominal current value may be twice what is actually needed.
Another limitation encountered when using a current reference is that an off-chip, precision resistor is typically required to generate the reference current. The off-chip resistor adds to the cost of each design which makes use of such a reference, and also requires physical real estate which might otherwise be available for additional circuit components and features.
Finally, standard off-chip current references require routing current to all locations where it is needed. If such routing is not desirable, multiple references must be used, further increasing cost and real estate requirements.
For these reasons, and others which will become apparent to those skilled in the art upon reading and understanding the instant specification, there is a need in the art for a current reference with reduced sensitivity to voltage, temperature, and process variations. Such a reference should also eliminate the need for an off-chip resistor as part of its operational circuitry.